1. Field of the Invention
The present invention relates to a grinding wheel and a method for manufacturing the same, especially to a grinding wheel effectively utilizing abrasive grains without causing partial abrasion and a method for manufacturing the same.
2. Description of the Prior Art
Hitherto, there has been proposed a grinding wheel to be mounted on a grinder, etc. for grinding a metal or non-metal material such as described in TOKKYO KOHO No. 50-35270. Such a grinding wheel, which includes an outer flange section and an inner offset section protruding to have a trapezoid shape, is formed of an abrasive which is made by bonding rough abrasive grains with one another using phenol resin type binder. Moreover, a reinforcement, which is made by immersing denatured phenol resin or denatured epoxy resin in plain-woven fabric made of glass fiber, is buried in the grinding wheel. In addition, in order to impart flexibility to the grinding wheel, grooves are formed in a grid-like manner on the grinding surface of the grinding wheel so as to be cracked at the bottom.
However, such a grinding wheel is gradually worn from the periphery to reduce the diameter. As a result, the circumferential speed of the grinding wheel is lowered; that is, the hardness of the grinding wheel against an object to be ground is lowered.
Consequently, the abrasive grains are rapidly peeled off. Therefore, when the grinding wheel is worn to some degree, it is required to be exchanged with new one.
Since such a conventional grinding wheel is formed of rough abrasive grains evenly, there arises a problem that all of the rough abrasive grains are not utilized effectively. That is, the grinding wheel is conventionally exchanged with new one even though some of the rough abrasive grains remain unworn at an inner portion thereof. For example, a case of a grinding wheel having an outer diameter of 100 mm, a thickness of 3 mm, and a hole diameter of 15 mm will be described. When assuming that such a grinding wheel is exchanged with new one after 30% of the total abrasive grains are worn, only about 50% of the total abrasive grains are actually used for the grinding process, and the remaining abrasive grains are uselessly discarded.
In addition, the grooves are formed in the grid-like manner on the grinding surface of the grinding wheel so as to be cracked at the bottom, thereby imparting flexibility to the grinding wheel.
However, the grooves are likely to be cracked unevenly. Moreover, the density of the fabric used as the reinforcement cannot be uniform in the circumferential direction, so that the elasticity of the grinding wheel cannot be uniform either in the circumferential direction. As a result, the grinding wheel cannot readily be flexed in a weft direction nor a warp direction, but it is relatively readily flexed in other directions, especially in a diagonal direction connecting junctions of the weft and warp. Due to such a difference in elasticity, vibration called "tapping" occurs during the grinding process. That is, the portion of the grinding wheel readily being flexed is negatively contact with the object to be ground, so that it is worn slowly. On the other hand, the portion not being readily flexed is positively contact with the object, so that it is worn rapidly. Thus, the conventional grinding wheel has a problem of partial abrasion.
To overcome the problem in that the grinding wheel is inevitably flexed unevenly, there has been proposed a grinding wheel in which an abrasive layer is formed so that the thickness thereof is made thinner in a direction from the outer flange section to the inner offset section, while a backing layer is formed so that the thickness thereof is made thicker in the same direction, such as described in KOKAI TOKKYO KOHO No. 2-250775. However, in a case where a reinforcement is included in the above grinding wheel, "tapping" still occurs due to a difference in elasticity of the grinding wheel in the circumferential direction. That is, even the above grinding wheel cannot solve the problem of partial abrasion.